Genes contributing to reproductive success in laboratory mice have provided important insights into the molecular, developmental and physiological processes underlying mammalian reproduction and have served as models for studies of human infertility. Traditional strains of laboratory mice, however, are limited in the degree of variation in reproductive traits compared to the extent of variation observed in nature. In contrast, a close relative to the laboratory mouse, deer mice in the genus Peromyscus, exhibit striking differences in reproductive anatomy, sperm production, morphology and motility among species. This variation is due to the extreme divergence in mating system within the genus. In species in which females mate multiple times over a breeding season, there is intense competition between ejaculates of different males for fertilization of her eggs. Accordingly, there is strong selective pressure on male reproductive traits that improve fertilization success in promiscuous species; in closely-related monogamous species, however, selection is relaxed. Thus, the diversity of reproductive traits in Peromyscus makes them a valuable model for studies aimed at understanding the genetic basis of male fertility but, in addition, they also offe an entirely new perspective on gametic interactions. When sperm are released from these mice they form cooperative units-multiple cells form groups within the female reproductive tract, which enable them to swim with greater motility compared to individual sperm. In at least one species, the species in which sperm competition is most intense, sperm are able to recognize the most related cells and selectively group with them; in fact, this form of cellular recognition s so refined that sperm from one male can even discriminate against sperm from full sibling littermate. In contrast, sperm from a monogamous species group indiscriminately. The proposed study is designed to exploit the natural variation in male reproductive traits as well as the uniqu cellular recognition and aggregation behavior of Peromyscus sperm to reveal the genes that contribute to fertilization success. The primary goal in the mentored phase of this project is to identify genetic regions and ultimately genes influencing a morphological trait of sperm that is associated with motility and reproductive success using a genetic mapping approached combined with gene expression studies of the testicular tissue that represent different stages of spermatogenesis. During the independent phase of this project the focus will be on exploring sperm aggregation behavior to understand both how groups form using integrative electron microscopy and why they do - by asking what is the effect of cooperative sperm migration within the female reproductive tract and in complex environments? Finally, with an understanding of the physical mechanisms involved in sperm aggregation, this study will apply similar genetic and genomic techniques implemented in the mentored phase to reveal the genetic basis of sperm aggregate size and the molecular mechanisms involved in cellular recognition, discrimination and adhesion in sperm. In total, this work will shed new light on the genetic basis of traits associated with male fertility and offer a unique perspective on gametic recognition and adhesion. PUBLIC HEALTH RELAVANCE: Understanding the genetic basis of male reproductive traits will provide important insights into infertility and sub fertility, a problem encountered by 1 in 1 couples. Moreover, a careful mechanical, physiological and molecular characterization of selective sperm adhesion using a novel approach will inform our understanding of gamete recognition, adhesion and communication. This work focuses on naturally variable phenotypes in an emerging rodent model, Peromyscus mice, which adds a new perspective to fertility studies commonly performed in laboratory mice, which do not show the extreme natural phenotypic variation or selective adhesion that Peromyscus sperm do, or in humans, where such controlled experiments are not possible.